Although lithium-ion batteries are robust and highly safe, they must not be overcharged, over-discharged, or overheated. To extend the service life of batteries and ensure user safety, it is necessary to build a battery management system (BMS) to make sure that the batteries always operate safely. The main function of the BMS is to measure a cell's voltage and protect the cell. When it comes to a series-connected battery, its cells differ in terms of voltage, because the cells have different internal resistance levels or undergo different processes. As a result, the performance and service life of the series-connected battery will be greatly reduced, if one of its cells is overcharged or over-discharged.
In practice, there is always a tiny difference in the internal resistance and capacity between the cells. The difference not only increases with the number of charging-discharging cycles over time, but also develops between battery modules as a result of poor management. Regarding the latter consequence, inconsistency of the battery modules causes overcharging and over-discharging to the battery earlier than expected. The inconsistency, whether attributed to the cells or the modules, must be corrected with an appropriate management system in order for the battery to function well. To this end, a balancing mechanism is required for the battery. Hence, both the BMS and the balancing mechanism are of vital importance.
Regardless of its performance, a conventional management system is always built in any low-voltage apparatus. For instance, sophisticated battery management technology is applicable to a wide variety of handheld apparatuses. IC manufacturers, such as Texas Instrument, Linear Technology, and Maxim, manufacture commercial chips dedicated to battery management ICs and widely applicable to handheld electronic products. However, the dedicated battery management ICs are usually intended for batteries with a low capacity and a few series/parallel connections but seldom come with a built-in cell balancing mechanism.
Issues arising from an unbalance between cells or between modules are no longer negligible whenever batteries are upgraded and applied to large power storage applications like a high-voltage energy-storing system or an electric bus. Take electrically-driven vehicles as an example, the most notorious drawbacks include high battery prices and short service life; the drawbacks are there, because conventional battery management technology is rarely effective in ensuring a balance between cells or between modules. As a result, the riskiest cell is always the first one to trigger management system protection earlier than expected and thus suspends its operation, thereby deteriorating the performance of the battery. Furthermore, the management system might fail to protect the riskiest cell timely, and thus the riskiest cell ends up being overcharged or over-discharged, thereby shortening the service life of an energy-storing device.
Conventional uniform battery balancing is of two types: active balancing and passive balancing. A cell balancing mechanism is one of the important factors in extending battery endurance. Primitive battery management systems are based on passive balancing.
Passive balancing works by lengthy power consumption of a series-connected battery without taking the initiative in increasing the power level of the cell with the lowest voltage or power level in the series-connected battery, as its name suggests. Although passive balancing requires a simple circuit and incurs low costs, passive balancing is disadvantaged by its low efficiency and lengthy balancing process, thereby rendering it inapplicable to high-capacity batteries.
Active balancing involves charging a cell of a battery whenever the cell is operating at the lowest possible voltage, so as to increase the voltage of the cell to a level as high as another cell operating at the highest voltage, and then performing the aforesaid process on the other cells until the voltages of all the cells are balanced. Depending on its operating principle, active balancing falls into three categories: inductive balancing, capacitive balancing, and multiple winding transformer balancing.
Both inductive balancing and capacitive balancing require a series-connected battery connected to a uniform balancing secondary circuit in parallel. The uniform balancing secondary circuit comprises inductors or capacitors and switches. The cell with the highest voltage or power level is detected, and then the electrical energy of the cell thus detected is stored in the inductors or capacitors. Afterward, the uniform balancing secondary circuit is switched by a switch circuit to a cell with the lowest voltage or power level. The aforesaid steps are repeated until all the cells have their voltages or power levels uniformly balanced. However, both inductive balancing and capacitive balancing have disadvantages as follows: the energy conversion is limited by the capacity of the inductors or capacitors; balancing takes much time; inductive balancing and capacitive balancing are not applicable to scenarios where lithium-ion batteries allow high-current charging-discharging.
Multiple winding transformer balancing requires a transformer with multiple secondary windings and each winding externally connected to a switch circuit. Multiple winding transformer balancing not only entails charging a series-connected battery but also enables a switch to charge a cell with a low voltage or power level. The aforesaid charging performed by multiple winding transformer balancing is deemed constant voltage charging, because multiple secondary windings always require the same number of windings. Furthermore, when all the control switches are ON to charge all the cells, the battery are charged to a lesser extent when having a high power level, but to a larger extent when having a low power level; hence, it does not take much time charging before a balance is attained. But, in practice, coupling winding is accompanied by mutual induction and induction leaks, and in consequence it is impossible for the cells to acquire the same voltage level despite the same number of windings, not to mention that the multiple windings take up much space.
Intensive series-connection applications usually necessitate distributed battery management by modularization. Even if conventional battery management systems are capable of balancing the cells of each module, conventional battery management systems cannot ensure a balance between the modules. Inconsistency of the modules inevitably brings about inconsistency of battery operating states, thereby rendering module balancing futile.